1. Field of the Invention
The present invention relates to an apparatus for computing the amount of intake air to be supplied to the cylinders of an internal combustion engine. More particularly, this invention relates to an apparatus which computes the amount of intake air in an internal combustion engine having a valve timing mechanism that can change the operational timing of at least one of an intake valve and an exhaust valve.
2. Description of the Related Art
There is an internal combustion engine having a valve timing mechanism which can change the operational timing of the intake valve or exhaust valve. A variable valve timing mechanism (hereinafter abbreviated as VVT) is used to obtain the flat output torque characteristics over the entire operation area of the internal combustion engine or to stabilize the number of the rotations of the engine (engine speed) at the idling time.
Japanese Unexamined Patent Publication No. hei 3-3910 discloses a fuel injection amount control apparatus for an engine which has this type of VVT. The VVT described in this publication is designed to switch the operational timing of at least one of the intake valve and exhaust valve between a first timing suitable for a low engine speed and a second timing suitable for a high engine speed.
In FIG. 15, a characteristic curve L1 indicates the output torque of an engine when the first timing is selected, and a characteristic curve L2 indicates the output torque of the engine when the second timing is selected. The operational timing of a valve is switched at the point where those output torques coincide with each other, as indicated by a point NE1 in FIG. 15.
To effect this switching, an injection amount map for the first timing indicated by a characteristic curve L3 and an injection amount map for the second timing indicated by a characteristic curve L4 are prepared previously. Those maps are prepared in consideration of the intake air amount and fuel injection amount that affect the output torque of the engine. More specifically, for each valve timing, the intake air amount corresponding to the engine speed NE and the pressure PM in the intake manifold are previously obtained through experiments and the injection amount according to the intake air amount necessary to obtain a predetermined air-fuel ratio of an air-fuel mixture (ratio of the weight of air to that of the fuel) is set.
The valve timing is switched at the point NE1 where the injection amounts on the two injection amount maps almost coincide with each other. This scheme intends to suppress the shock from a torque change at the time the intake valve timing or exhaust valve timing is switched.
In a conventional engine having a mechanism which can set the valve timing as desired with respect to the strokes of pistons, the charging efficiency, having a close relationship with the intake air amount, changes in accordance with the running conditions of the engine. The charging efficiency is the ratio of the actual weight of air supplied to each cylinder to the weight of air to be supplied to each cylinder under atmospheric pressure. This charging efficiency changes in various forms in accordance with the closing timing of the intake valve and the length of the period the intake valve and exhaust valve are opened, i.e., the so-called valve overlap period. This charging efficiency is also influenced by the so-called internal exhaust gas recirculation (hereinafter abbreviated as internal EGR) by which a part of the burnt gas in a combustion chamber is mixed with the air-fuel mixture and recirculates. The amount of the recirculated gas is influenced by the pressure at the downstream side of the exhaust valve in the exhaust passage (nearly equal to the atmospheric pressure) and the intake pressure.
In view of the above, it is difficult to perform highly accurate injection amount control over the entire operation area of the internal combustion engine by simply switching the injection amount maps for the individual timings without considering a change in the charging efficiency.